1. Technical Field of the Invention
The subject matter of this invention finds use in intermittent towing operations by seismic recording ships working in shallow water and as an attaching means for decoupling the cable mechanically from the vessel's motion.
2. Discussion of the Prior Art
In marine seismic surveying, a ship tows a streamer cable through the water along a line of survey. The streamer cable contains a plurality of seismic sensors. A corresponding plurality of conductor pairs are included in the streamer cable to electrically interconnect the sensors with signal processing equipment aboard the ship. At specified recording stations along the survey line, a seismic sound source is fired. The resulting acoustic pulse travels downwardly through the water and through the rock layers therebeneath. At each rock-layer interface some of the acoustic energy is reflected back to the surface of the water where the reflected energy is detected by the sensors.
The streamer cable is connected to the ship by a lead-in cable. Whereas the streamer cable is neutrally bouyant and includes sensors, electrical conductors and stress members, the lead-in cable includes only a single, heavy stress member and the electrical conductors. Usually, the lead-in cable is several hundred feet long.
In deep water, the streamer cable is towed continuously through the water at constant speed. Pressure-actuated diving modules hold the cable at a constant desired depth in the range of 20-40 feet. Since no appendages are attached to the streamer cable there is very little towing noise.
In shallow water, it is impractical to maintain the streamer cable at a constant depth by means of automatic diving modules. Accordingly, the streamer cable is suspended from surface bouys by suspension lines. Alternatively, the bouyancy of the cable may be increased and weights may be attached thereto to hold the cable a fixed distance from the bottom; three or four feet above the bottom is common. When towed through the water the attached bouys and suspension lines create considerable towing noise. Accordingly, at each recording station, the streamer cable must be brought to a standstill. It is impractical to stop the ship at each recording station. Therefore an intermittent towing mode is used as described in a copending application Ser. No. 162,865 entitled Marine Seismic Cable Handler, Filed June 25, 1980 and assigned to the assignee of this invention. A similar system is described in U.S. Pat. No. 3,286,225 to Huckabay et al.
For intermittent towing, a yo-yo reel is provided, that is separate from the usual cable-storage reel. A separate tether line is secured to the streamer cable and is reeled in by the yo-yo reel. At a recording station, the reel is released so that the tether line unreels and allows the streamer cable to drift to a stop behind the ship as the ship moves forward at constant speed. After data have been recorded, the tether line is reeled in again in anticipation of the next recording station.
Since the lead-in cable is several hundred feet long and since the tether need by only between 100 and 200 feet long, the tether line is attached to the lead-in cable by a suitable saddle clamp. When the tether line is reeled in, the excess lead-in cable is doubled back on itself at the saddle clamp. Each time that the tether line is payed out, unavoidably the lead-in cable flexes sharply at the saddle clamp. After a period of time, the electrical conductors work-harden and break. Repair of the lead-in cable is laborious and expensive.